1. Field of the Invention
Embodiments disclosed herein generally relate to methods and systems for designing and forming magnetic coils.
2. Description of the Relevant Art
Magnetic resonance imaging (MRI) systems use a combination of magnets and other equipment to produce nuclear magnetic resonance (NMR) images of an object, such as biological tissues. Typically, an MRI system includes a main magnet that produces a large, uniform, homogeneous magnetic field over a region of interest (ROI). The main magnetic field lies along a single axis, commonly referred to as the “z” axis. This magnetic field may thus be referred to as the Bz (or, alternatively, the Bo) field. The MRI system further includes gradient coils that produce orthogonal gradients in the Bz field. Typically, the gradient coils are designed to produce a linear variation in the Bz field in the relevant direction (i.e., x, y, or z) over the region of interest. These gradients may be used to establish location within the region of interest. The MRI system additionally includes radio frequency coils that may, among other things, detect magnetization characteristics (e.g., spin) in the imaged object.
Another application for magnetic coils is in magnetic stimulation. Magnetic stimulation involves the induction of electric fields in a nerve or neuron through the pulsing of a large magnetic field. Transcranial magnetic stimulation involves the application of such a field to the head of a subject which crosses through the skull and excites neurons in the brain, thereby creating “virtual lesions” within the brain. It may be desirable that the excited region be as focused as possible, while maintaining penetration. Some advantage may be obtained through curving a coil structure to the shape of a head.
A number of methods are known for designing magnetic coils for MRI and other systems. Many previous coil designs rely on simple geometries that have analytical solutions, the most common of which may be target field method and variations thereon. Some target field methods are described in Turner [1]. Target field methods have been used for cylindrical and planar designs.
Some methods for designing magnetic coils involve the use of numerical methods to optimize the positions of the conductors in the coil. For example, Crozier et al. [2] describe a simulated annealing algorithm to develop asymmetric gradient coils of elliptical cross section using a set curve structure. Tomasi [3] describes a method of simulated annealing to optimize a stream function given by the target field method to design self-shielded gradient coils of cylindrical geometry.
Access and comfort may be of great importance with routine use of MRI in clinical studies. The development of radio frequency (rf) and gradient coils having open geometry (e.g., where the coil is partially open in at least one transverse direction) may be an important step in improving access and comfort. For example, an open coil may facilitate functional imaging of a primate brain while allowing for freedom of movement of the limbs. As another example, open coil geometry may facilitate imaging of a portion of a limb, such as a forearm or wrist.
In addition, an open gradient coil set, in combination with radio frequency coils of similar geometry, may enable easier access to a patient. Open coil sets may also alleviate some claustrophobia concerns.
It is therefore desirable that new methods be developed for designing magnetic coils.